The control system according to the present invention may be used in general for controlling operation of a machine. In the following, the invention will, however, be exemplified by reference to a steering system.
Ships and vehicles, and in particular off-highway machinery such as wheel loaders, excavators, dozers, articulated vehicles, tractors, harvesters and similar heavy duty machines often operate with hydraulic, electro-hydraulic, and electric steering systems. Typically, the steering system receives a desired input to indicate a desired angular movement of the ship or vehicle. The input could be provided via input means such as a joystick or a steering wheel, or the input could be derived from a GPS system or similar system for positioning and tracking. The input is converted into a signal for an actuator, e.g. a hydraulic cylinder, which moves a steered element, e.g. a wheel of a vehicle or a rudder of a ship. Steering systems are disclosed e.g. in U.S. Pat. No. 6,668,967 and in EP0856453.
In a fully hydraulic steering, the actuator is typically supplied with a hydraulic flow via a steering unit which is connected to the steering input means. The steering unit often comprises a spool which is moved based on the position of the input means. The spool opens or closes a path from a source of a hydraulic fluid under pressure to one out of two hydraulic ports of the actuator. A fluid flow into one of the ports and out or the other port forces the actuator to move in one direction and a fluid flow in an opposite direction forces the actuator to move oppositely, e.g. right or left. Typically, the input means is a steering wheel or a joystick. The input means is movable between a centre set-point and set-points on opposite sides of the centre set-point, e.g. corresponding to a right turn or a left turn.
In steering systems and other control systems, reliability is important. An incorrect electrical signal or fluid flow may cause wrong steering and thus be dangerous. In order to increase reliability, several control systems are sometimes connected in parallel. If one system faults, another one will take over. This, however, requires fault detection and switching between parallel systems.